What could be the cause of this issue?
The management/payer account does not have billing alerts turned on.
The company has not configured AWS Resource Access Manager (AWS RAM) to share billing information between the member accounts and the management/payer account.
Amazon GuardDuty is turned on for all the accounts.
The company has not configured an AWS Config rule to monitor billing.
Explanations:
The management/payer account needs to have billing alerts turned on for billing alarms to work across all accounts in the organization.
AWS Resource Access Manager (AWS RAM) is not necessary for sharing billing information; billing information is automatically available in the management account.
Amazon GuardDuty has no effect on billing alarms or notifications. It focuses on security monitoring, not billing.
AWS Config rules monitor resource configurations, but do not track or monitor billing information. Billing is managed via AWS Cost Explorer and alarms, not AWS Config.
As far as I’m aware, the answer is:
The management/payer account does not have billing alerts turned on.